It is well-known in civil engineering practice to employ prestressing steel elements to reinforce concrete structures such as buildings, bridges, graded slabs, pavements and the like. In a post-tension concrete structure, the prestressing steel elements are disposed in a desired array prior to concrete pouring. Once a concrete member has been poured and has sufficiently cured, the prestressing elements contained within it are tensioned, and a prestressing force is thereby transmitted to the structure by means of mechanical anchorages provided at the ends of the elements and secured to the concrete member.
In an unbonded post-tension structure, the prestressing steel elements are intended on a permanent basis to move freely relative to the concrete substrate in which they are placed. Prestressing steel elements in unbonded structures may typically be found in the form of strands, cables, wires or bars. To prevent such prestressing steel elements from bonding to the concrete, they are typically enclosed in a conduit means such as a sheath of plastic or metallic tubing. In addition, the prestressing elements are usually covered with a corrosion-protective coating, which may be in the form of corrosion inhibiting greases or other coatings such as bitumastics, wax, epoxy or zinc.
It has been estimated that there exists more than 2,000,000,000 sq. ft. (185,000,000 m.sup.2) of unbonded post-tension structures in the United States and Canada, with a large proportion of this construction being represented by parking structures. This may be attributed to some of the recognized or perceived advantages associated with unbonded post-tensioning, namely the attainment of long span designs, relatively shallow slab depths and improved crack control. However, despite these advantages, it has been found in practice that unbonded post-tension concrete structures have been prone to corrosion and failure of the prestressing steel elements.
Entrainment of water and corrosive contaminants within a post-tensioned concrete substrate can occur as a result of a number of design and construction defects such as inadequate concrete cover, poor-quality concrete, concrete cracking, pour concrete surface sealing practices and inadequate end anchorage protection of the prestressing elements. A prestressing element with a discontinuous grease coating, poor or defective quality grease or a loose fitting sheath will result in such entrained water and chemical contaminants coming into direct and prolonged contact with the prestressing element, thereby rendering it susceptible to corrosion.
The susceptibility of unbonded prestressing elements to corrosion may have serious effects for the long term durability and structural soundness of post-tension concrete structures. For instance, failure of a prestressing element due to severe enough corrosion may lead to eruption of the prestressing elements through the concrete cover or may even cause the tensioned prestressing elements to suddenly rupture and eject out of the structure through the end anchorages of the elements. It will be readily appreciated that both of these forms of projection of prestressing elements from the concrete structure can pose serious safety hazards. For instance, when a ruptured prestressing cable erupts through its concrete cover, it may do so with an explosive force, thereby having the potential to cause serious personal injury and property damage.
Moreover, where enough prestressing elements in a given concrete member have failed, structural deficiency may result if such unsound or disactivated prestressing elements are not located and repaired in a timely manner. Those skilled in this art will readily appreciate that since prestressing elements in an unbonded structure are only secured at their end anchorages, a single failure location along the entire length of a prestressing element will be sufficient to render the entire element ineffective for providing structural integrity. As well, any significant proportion of damage to the prestressing elements in any one area of a post-tension structure may have important consequences on the load-carrying or load-bearing capacity of the structure.
As mentioned previously, the use of post-tension structures has been prevalent in automobile parkades. In some of these structures, the concrete cover is exposed to the outside elements and in others, de-icing agents containing chlorides routinely come into contact with the concrete cover. Although the prestressing elements of the parkade structures may have been originally designed and intended to be protected from corrosion by a grease or other corrosion inhibiting coating applied to the prestressing elements at the time they are installed into their sheaths, such coatings have been found on occasion to be incomplete over the length of the element. In other instances, the coatings may emulsify or become dispersed over time by water infiltration or may otherwise be rendered ineffective. As well, even in cases where the concrete cover was initially treated to prevent water infiltration, poor maintenance or surface erosion caused by snow removing operations may subsequently render the concrete permeable to water. In such cases, infiltration of water into the concrete substrate, which may moreover contain chlorides from de-icing agents, may induce surface pitting, stress corrosion cracking and hydrogen embrittlement of the steel prestressing elements.
Where the rehabilitation of a post-tension structure calls for replacement of its prestressing elements, such repair work may result in an expense of several million dollars for a multi-storey structure containing 2,000 to 3,000 prestressing cables. However, in some cases, such an extensive degree of damage may have been caused to a post-tension structure that repair and rehabilitation of the original prestressing array is no longer technically nor economically feasible. It may therefore be appreciated that there exists a need for non-destructive methods of evaluating the corrosion condition of prestressing arrays and of protecting them from continued corrosion.
Currently, there does not appear to be a reliable, relatively inexpensive method for evaluating the condition of unbonded prestressing elements which are in service and therefore tensioned within a concrete substrate. One known method of evaluating the condition of prestressing elements is to extract a statistically relevant portion of the prestressing elements in a given structure and inspect them visually or by known instrumental testing methods. This method may also be coupled with an exploratory excavation into the concrete substrate to expose a desired additional number of prestressing elements for on-site visual examination. Based on such a method, it may be possible to statistically determine the degree of deterioration for the entire structure and take corrective measures to prevent or retard corrosion. However, not only is this approach costly, but it also requires a high degree of professional engineering judgment.
Another known method for the on-site evaluation of a prestressing cable in service involves applying a localized vacuum over the end anchorages of the prestressing element and observing both the level and rate of decay of the maximum vacuum pressure obtained. Such a method can be used to obtain an indication of the concrete plug joint tightness and relative porosity of the concrete in the vicinity of the end anchorage plug of a prestressing cable. This particular method cannot therefore be used to evaluate the condition of a cable along longitudinal portions thereof which are far removed from its end anchorages.
Conventional load testing of post-tension slabs may also be attempted to evaluate the condition of the prestressing elements, but such load testing does not necessarily provide reliable information about whether or where the prestressing elements are corroded, nor any indication of whether any on-going corrosion activity can be expected to continue to reduce the slab load bearing capacity. Moreover, conventional load testing is both costly and time consuming.
Finally, methods employing either ultrasonic or acoustic emission techniques or electrical conductivity techniques have been proposed for determining the structural soundness of prestressing elements, but some have found that such methods are not necessarily workable in that a reliable correlation between the test results produced by these methods and the actual corrosion condition of the prestressing elements is difficult to ascertain.
None of the prior art methods discussed above for the on-site evaluation of unbonded prestressing elements is suitable nor easily adaptable for the treatment and corrosion protection of prestressing elements once they have been evaluated. Thus, the known methods of evaluation cannot be resorted to for restoring a prestressing element to an acceptably dry condition nor for protecting the restored element from further corrosion.
It is therefore one object of the present invention to provide a relatively reliable and cost-effective method for the non-destructive condition evaluation of unbonded prestressing elements in post-tension concrete structures, so as to enable a determination of the corrosion condition of such prestressing elements and their susceptibility to further corrosion.
It is another object of the present invention to provide a method and apparatus for the evaluation of prestressing elements by sampling and assessment of the gaseous environment contained within the conduit means of the prestressing element.
It is a further object of the present invention to provide a said method and apparatus which will also serve in the treatment of the gaseous environment within the conduit means of prestressing elements by removing moisture, bulk water and other impurities conducive to corrosion from within the conduit means.
It is yet another object of the present invention to provide the said method and apparatus which will also enable the detection of leaks and probable points or areas of moisture and corrosive agent penetration in the concrete cover and anchorage ends.
It is yet another object of the present invention to provide a said method and apparatus which facilitate long-term maintenance and protection of the prestressing element environment by continuous or cyclic pressurization and replenishment of the gaseous environment within the conduit means of the prestressing element with a dry non-corrosive gas.
It is also an object of the present invention generally to attempt to overcome the problems and deficiencies occasioned by the prior art methods of evaluating, treating and protecting unbonded prestressing elements in post-tension concrete structures.
These and other objects are sought to be attained by way of the present invention, as more fully described and illustrated herebelow.